Light emitted from natural light sources such as the sun or from artificial ones such as lamps is generally non-polarized (randomly polarized). A polarizing plate can convert such non-polarized light into polarized light (linearly, circularly or elliptically polarized light). The thus-converted polarized light can be used in various optical instruments. For example, a liquid crystal display widely used nowadays is an instrument that utilizes characters of polarized light to display an image.
The term “polarizing plate” in a broad sense includes linearly, circularly and elliptically polarizing plates. However, that term usually means (in a narrow sense) only a linearly polarizing plate. Also in the present specification, the simple “polarizing plate” means a linearly polarizing plate.
A linearly polarizing plate is a basic polarizing plate. For example, a circularly polarizing plate is generally a layered composition of a linearly polarizing plate and a λ/4 plate, and also an elliptically polarizing plate is generally a layered composition of a linearly polarizing plate and a phase retarder other than the λ/4 plate. Therefore, the optical character of linearly polarizing plate is important for various kinds of polarizing plates (in a broad sense).
A linearly polarizing plate generally used is a polarizing plate of light-absorbing type made of polyvinyl alcohol. For preparing the polyvinyl alcohol polarizing plate, a polyvinyl alcohol film is stretched and made to adsorb iodine or a dichromatic dye. The thus-prepared plate has a transparent axis (polarizing axis) perpendicular to the stretching direction.
The polarizing plate of light-absorbing type transmits only a light component polarized parallel to the polarizing axis, and absorbs a perpendicularly polarized component. Therefore, usable light through the plate is theoretically 50% or less (practically less by far) of the original light.
In order to increase the amount of usable light (efficiency of light), it is proposed that a polarizing plate of light-scattering type be used in place of or in addition to that of light-absorbing type. The polarizing plate of light-scattering type also transmits only the light component polarized parallel to the polarizing axis, but scatters forward or backward the perpendicularly polarized component. Consequently, the plate of light-scattering type improves the efficiency of light.
There are some mechanisms in which the polarizing plate of light-scattering type improves the efficiency of light. The mechanisms (A) to (C) are described below.
(A) Depolarization of Front Scattered Light
The polarizing plate of light-scattering type scatters forward or backward the light component polarized perpendicularly to the polarizing axis. The light component scattered forward (front scattered light) is depolarized to rotate the polarizing direction. The thus rotated direction is different from the polarizing direction of the incident light, and hence the scattered light has a light component polarized parallel to the polarizing axis of the plate. Consequently, the component polarized in the polarizing direction of the plate is increased. If the polarizing plate contains many particles in the thickness direction, multiple scattering occurs to enhance the depolarization. In this way, the efficiency of light is improved by the depolarization of front scattered light if the polarizing plate of light-scattering type is used, as compared with the efficiency when the polarizing plate of light-absorbing type is used alone.
(B) Reuse (Depolarization) of Back Scattered Light
The component scattered backward (back scattered light) is depolarized when it scattered. The back scattered light is reflected by a metal reflector placed behind the backlight (light source), and again enters the polarizing plate of light-scattering type. Since the depolarized light (back scattered light) has the light component polarized parallel to the polarizing axis of the plate, a part of the reentering light passes through the plate. The scattering on the polarizing plate and the reflection on the reflector are repeated in this way to improve the efficiency of light.
(C) Reuse (Rotation of Polarizing Direction) of Back Scattered Light
In an optical system comprising a λ/4 plate and a metal reflector, incident light linearly polarized at 45° to the slow axis of the λ/4 plate is reflected to rotate its polarizing direction by 90°. For applying this effect, a λ/4 plate is provided between the polarizing plate of light-scattering type and the metal reflector (placed behind the backlight) so that the slow axis of the λ/4 plate may be placed at 45° to the polarizing axis of the polarizing plate.
The light scattered backward has a component polarized perpendicularly to the polarizing axis of the polarizing plate in a large amount. Accordingly, the light having passed through the λ/4 plate, reflected by the metal reflector and reentering the polarizing plate has a component polarized parallel to the polarizing axis of the polarizing plate in a large amount. Since the light component parallel polarized can pass through the polarizing plate, the efficiency of light can be improved by the λ/4 plate provided between the polarizing plate of light-scattering type and the metal reflector.
The polarizing plate of light-scattering type is described in Japanese Patent Provisional Publication Nos. 8(1996)-76114, 9(1997)-274108, 9(1997)-297204, Japanese Patent Publication Nos. 11(1999)-502036, 11(1999)-509014, U.S. Pat. Nos. 5,783,120, 5,825,543 and 5,867,316.
The known polarizing plates of light-scattering type disclosed in Japanese Patent Provisional Publication Nos. 8(1996)-76114, 9(1997)-274108, Japanese Patent Publication Nos. 11(1999)-502036, 11(1999)-509014, U.S. Pat. Nos. 5,783,120, 5,825,543 and 5,867,316 are stretched polymer films like the known polarizing plates of light-absorbing type. In the polarizing plates, the refractive induces of the binder and the dispersed particles should be highly adjusted. In more detail, the difference between the refractive induces of the binder and the particles should be small at the side of transmitting polarized light. On the other hand, the difference should be large at the side of scattering polarized light. The known plates described in the prior art references are insufficient for the above-described optical characteristics. Therefore, the improvement of the efficiency of light is insufficient.
The polarizing plate of light-scattering type disclosed in Japanese Patent Publication No. 11(1999)-502036 is a polymer film containing dispersed liquid crystal molecules (optically anisotropic molecules), which are aligned by applying an electric or magnetic field. However, it is practically difficult to apply the electric or magnetic field evenly to a large area, and accordingly it is difficult to obtain an even scattering character in a large area. Unevenness of the scattering character in the plane of the polarizing plate induces unevenness of brightness in the plane of a liquid crystal display.